A material for diffusing light has recently found use in a variety of fields. For example, in the case of interior illumination, the quantity of emitted light is uniformized by passing the light through a diffusion panel instead of directly illuminating a room with a fluorescent lamp. In addition, a direct type backlight unit to be used as the backlight unit of a large liquid crystal television set is also converted into a uniform planar light source by placing a diffusion panel on a fluorescent tube to alleviate brightness unevenness.
A light diffusion material as exemplified by such diffusion panel produces a diffusion effect by adding materials such as fine particles having different refractive indices to a matrix to induce a light scattering phenomenon.
However, the light scattering phenomenon has wavelength dependency as typified by the blue sky, so a light scattering material involves the following problem: the wavelength dependency of a white light source changes, and white light is observed to be color unevenness.
JP 11-153963 A (Cited Document 1) proposes a diffusion material for improving the uniformity of a color tone to solve the problem. The diffusion material disclosed in Cited Document 1 is a light scattering light guide the inside of which is provided with scattering power by dispersing, in a medium having a predetermined refractive index (a matrix or a matrix), scatterers such as spherical fine particles each having a refractive index different from that of the matrix. The diffusion material is used in a light source device which: is provided with light supplying means for supplying visible light including a long wavelength region and a short wavelength region from a side end face portion of the light scattering light guide; and causes incident light to outgo from an outgoing surface formed in at least a side portion of the light scattering light guide. In addition, Cited Document 1 discloses that evaluation for the scattering power imparted to the light scattering light guide can be performed by using scattering efficiency Q(λ) (λ: wavelength) determined from the theory of Mie scattering. That is, the scattering power imparted to the light scattering light guide of Cited Document 1 is as follows: a relationship between scattering efficiency Q(R) in a long wavelength region (long wavelength visible region typified by a red color) and scattering efficiency Q(B) in a short wavelength region (short wavelength visible region typified by a blue color) is balanced so that the color temperature of outgoing light from the outgoing surface at a position close to an incident side end face portion and the color temperature of the light at a position distant from the position are substantially equal to each other.
In addition, Cited Document 1 further discloses that a control ratio k for balancing the scattering efficiency Q(λ) represented by a ratio between the scattering efficiency Q(B) in a short wavelength region and the scattering efficiency Q(R) in a long wavelength region is preferably adjusted to satisfy the following range:k=Q(B)/Q(R)0.75≦k≦1.25where B=435 (nm) (blue color) and R=615 (nm) (red color).
In addition, the document discloses that the shape of the light scattering light guide is preferably a plate shape, a wedge shape (which tends to be thinner as it is more distant from a side end face portion), or a rod shape.
As a result, in Cited Document 1, a problem concerning color unevenness occurring in a light source device using a light scattering light guide is solved, the nonuniformity of the color tone of outgoing light depending on whether the light is distant from or close to a light supply end is alleviated, and the uniformity of a color tone, for example, in the backlighting of any one of various displays such as a liquid crystal display or in any one of the other various lighting units can be easily improved.    Patent Document 1: JP 11-153963 A